What happened
On September 20, 1980, during the 7th International Alpine Hot Air Balloon Week, a Colt 77A hot air balloon, registration HB-BEX, departed from Flims-Unterwaldhaus for a low-altitude flight. The pilot, accompanied by two passengers, initially drifted steadily along the Flimserstein slope toward Bargis. The flight was intended to be a short-duration excursion following weather forecasts that predicted only weak winds.
As the balloon approached the Bargis area, the weather conditions changed abruptly. A sudden and violent burst of Föhn wind struck the aircraft. While flying just 2 to 3 meters above the ground, the balloon struck a transport cable that was suspended at a very low altitude. The impact was severe enough to dislodge the cable from its downstream anchor, causing the cable to be dragged along the slope.
As the cable was pulled, it rubbed against the structural load tapes between the balloon envelope and the basket. The friction was intense enough to saw through the 6 mm steel basket cables and several 3 mm envelope load tapes. This structural failure caused the envelope to separate from the basket. Consequently, the basket fell approximately 30 meters to the ground, resulting in three fatalities.
The investigation
The investigation examined the flight path, weather conditions, and the physical wreckage of the HB-BEX. Investigators found that while the initial weather forecast suggested manageable conditions, the actual local conditions experienced a sudden, heavy Föhn gust with peaks exceeding 45 km/h.
Technical analysis of the wreckage revealed that the transport cable, which was 117 meters long and heavily rusted, had been temporarily placed over a steel pipe rather than being properly anchored to its permanent steel beam. The investigation also noted that the pilot had closed the main gas valves during the collision, likely preventing a post-crash fire. The investigation confirmed the pilot was experienced and medically fit, and the aircraft was well-maintained with no pre-existing technical defects found.
Findings
- The primary cause of the accident was the collision of the balloon with a transport cable positioned just above the ground.
- A sudden, intense burst of Föhn wind contributed to the impact.
- The steep terrain in the direction of flight exacerbated the difficulty of managing the aircraft during the wind event.
- The instability of the transport cable, which was not properly anchored to its intended base, allowed it to be dragged and subsequently sever the balloon's structural components.